Although the invention is not limited to such applications, it will be more particularly described with reference to spacers used for keeping a space between two glass sheets in the production of field-emission screens, and therefore for keeping a space of limited thickness, generally of less than a few millimeters, over the entire surface of the glass sheets.
Such a configuration is widely sought for producing display screens, whatever the technology thereof. These may be field-emission display (FED) screens mentioned above, such as microdot screens, or plasma screens. Such a configuration may also be sought for producing vacuum glazing units or flat lamps. The expression “flat lamps” should be understood as encompassing lamps which may have a curvature over at least part of their surface, whatever the technology of these lamps.
This is because, in a screen of the field-emission type, a limited space in which a vacuum is created must be maintained between the two glass sheets. Deposited on the glass sheet forming the front face, which constitutes the anode, are the light-emitting elements. Placed on the rear face, which constitutes the cathode, are electron-emitting elements, the electrons being accelerated toward the light-emitting elements and thus exciting them.
It is known, as described in document WO-81/01910, that this type of screen, more particularly the outer glass sheet, that is to say the glass sheet which is seen by an observer, must have a high optical transparency.
The spacers for keeping a space between the two glass sheets must consequently be the least visible possible.
A process has already been proposed, according to document EP-0 627 389 A, for manufacturing a glass polyhedron in which a preform of polygonal cross section, advantageously polished on all its lateral faces, is firstly drawn and then cut into several rods, which rods, after having been joined together, are in turn cut to the desired length and then their ends are polished.
This technique is advantageous in so far as it allows, for the least cost, glass spacers to be produced to dimensions which are however quite difficult, each of these glass polyhedra having very small dimensions directly equal to those required by the intended application.
This is because, in the case of display screens, such as microdot screens, the spacers must be placed very precisely on separating or “black matrix” strips. These separating strips are provided either in one direction or in two perpendicular directions, in order to define the color pixels. The spacers must be placed so that they do not encroach on the pixel regions.
The spacers thus positioned on the separating strips must not be visible through the outer glass sheet, that is to say the glass sheet which faces the observer, especially in the case of a display screen.
Nevertheless, it is apparent that, when display screens are being used, the positions of the spacers become visible by the appearance either of bright areas or of dark areas around said spacers. Of course, in the particular case of display screens, this phenomenon disturbs the quality of the image and therefore cannot be tolerated. This brightening and/or darkening phenomenon around the spacers is known already and explained. This phenomenon is in fact due to the implantation of charges in the spacer because of the secondary emission coefficient of the material, said coefficient being defined by the ratio of the number of secondary electrons re-emitted to the number of primary electrons received; a coefficient differing from 1 results in a local charging effect which, depending on whether the charge is positive or negative, results in a brightening or darkening effect due to the deviation of the electron path.
Spacers made of glass with standard compositions, exhibiting ionic conduction, do not allow electronic charges to be removed. Furthermore, these compositions may include elements which readily migrate under the influence of an electric field or owing to the temperature. It is therefore possible to reveal the migration of constituent elements from the glass matrix during, for example, the production of a screen because of the thermal cycles or else during its use because of the electric fields which then exist. However, the migration of these elements may result in contamination of, for example, the microdots in the case of this type of screen.
One solution, already proposed, for avoiding this drawback associated with a local charging effect consists in producing a surface coating on the spacer in order to obtain electronic conduction. This type of solution has the drawback of being very expensive since it requires the spacer to be treated after it has been manufactured. Furthermore, in the case of spacers of complex shapes, the layers are difficult to produce homogeneously, especially in terms of thickness, and may lead to irregularities in charge removal; again, this results in the risk of a breakdown.
The inventors were thus tasked with producing spacers which fulfil their function of keeping, for example, two flat substrates apart and which remain barely visible during the use of the product for which they are used, without having the drawbacks, especially in terms of costs, of the existing solutions.